Orthomode transducers (OMT) are of great interest in various applications as they enable to combine or separate two signals in orthogonal polarizations.
In telecommunications, for instance, these components permit an efficient use of the available bandwidth. In radar applications, one may use these components to separate the transmitted and received signals if they are in orthogonal polarizations.
These components are also of interest in measurement setups like compact range, long range or near field, as two orthogonal linear polarizations can be measured simultaneously.
A very convenient base to design a waveguide technology OMT is a turnstile junction as this component has wide band high power handling behavior.
FIG. 1 illustrates a conventional turnstile junction.
It consists in a circular main waveguide 10 and four secondary rectangular waveguides 11-14 that lie in a common plane along the orthogonal main axis AA′, BB′ of the junction. The turnstile junction 1 can be seen as the superposition of two H-plane power dividers with a 90 degrees rotation.
Used as an antenna feeding network, the circular main waveguide 10 is connected to the antenna port.
Depending on the use of the antenna (for transmit or receive), the circular main waveguide 10 is considered as an output or an input and accordingly the OMT combines or separate orthogonal polarizations.
To simplify the description, let assume in the following description that the OMT is used to separate two orthogonal linear polarizations received by the antenna.
A radio-frequency signal including a vertically polarized mode 20V and a horizontally polarized mode 20H enters in the main circular waveguide 10 of the junction according to the orientation defined by the main axis AA′, BB′ of the junction 1.
The vertically polarized field 20, is divided into two out-of-phase signal portions 21a and 21b that exit the junction by the two opposed ports 11 and 13 respectively. Similarly the horizontally polarized field 20H is divided into two out-of-phase signal portions 21a and 21b that exit the junction by the two other opposed ports 12 and 14 respectively.
With this junction, a linearly polarized electromagnetic field is naturally directed towards the rectangular waveguides 11-14 having the same axis direction.
Then, each pair of opposite waveguides needs to be recombined through a power divider/combiner. But due to the particular geometry of the turnstile junction, radio-frequency paths crossing usually lead to a large, non symmetrical geometry network such as the one described in document “A Turnstile Junction Waveguide Orthomode Transducer,” A. Navarrini et al., IEEE Transactions on Microwave Theory and Techniques.
The latter characteristic may have an impact on bandwidth performances and also on higher order modes generation.
Some solutions to overcome these drawbacks are already known.
One solution using waveguide cross-section reduction is described in document U.S. Pat. No. 7,330,088. This leads to a very compact symmetrical design. However cross-section reduction is known to limit power handling which is of great concern for telecommunication applications since the current trend is to increase the transmitted power per antenna.
Another solution is described in document WO 2008/008702. This design uses four magic tees to suppress radio-frequency path crossings. However this design leads to a combination network that requires three components per radio-frequency path compared to only one with previous designs. This may result in increased insertion losses and higher sensitivity to manufacturing precision.